The present invention relates to a process and an apparatus controlling the amount of thermal energy fed to the bottom of an extraction column for separating mixtures of materials.
Extractive distillation is a widespread practical and useful process for separating mixtures of materials and in particular of hydrocarbons, which cannot or can only partially be separated by distillation based on the boiling points of their components. In contrast to the liquid-liquid extraction frequently employed for separations of this nature, extractive distillation exhibits a number of advantages relating to apparatus construction and process engineering. For example, extractive distillation requires only two distillation columns. The solvent employed in extractive distillation is generally anhydrous. This eliminates the requirement of separate water circuits. Furthermore, in extractive distillation the viscosities of the extractant are lower based on the higher employed temperatures and this improves considerably the mass transfer between the extractant and the material to be extracted. This results in an improved loading and for the same throughput, smaller amounts of extractant are sufficient. The obtainable advantages in apparatus construction result in considerably smaller capital costs for an extractive distillation plant compared to those of a liquid-liquid extraction plant. The operating costs are also lower and are sometimes only about fifty percent of those of a corresponding liquid-liquid extraction plant.
In liquid-liquid extraction the formation of two liquid phases is a precondition for successful separation of the starting materials. Ideally, one phase of the liquid-liquid extraction process consists of the extractant and of the components of the extract and the other phase consists of the components of the raffinate. It is however frequently necessary in liquid-liquid extraction to add water to the extractant for improving the selectivity and for favoring the formation of two liquid phases. Adding water results in the requirement of separate water circuits which contributes to the increase of the capital costs of a liquid-liquid extraction plant.
The situation for extractive distillation is completely different. The separating effect is based in this case on the change of the vapor pressures of the individual components present in the mixture to be separated in the presence of the extractant. The changes are in the direction as to increase the vapor pressure differences between the components to be separated into either the extract or into the raffinate. Thus the raffinate can be distilled off at the top of the extractive distillation column as the lower boiling fraction. All deviation from normal of the streams of thermal energy fed to the extractive distillation column effect the top product forming the raffinate and the composition of the raffinate can vary considerably in its composition depending on the amount of heat fed to the extractive distillation column and on the composition of the mixture to be separated. In general there is found an increase in the amount of the components of the extract in the raffinate.
The practical consequences of this situation can be gathered from the following example relating to the extractive distillation for separation of aromatic hydrocarbons from varying starting materials. As is known, the desired aromatic compounds are removed at the bottom of the column together with the extractant as the extract and the nonaromatic compounds are removed at the top of the extractive distillation column as the raffinate. Taking as the starting point that the raffinate contains in each case still about 20 weight percent of aromatic compounds, which is the situation in usual cases wherein the heat input of the extractive distillation column is controlled according to the state of the art, then the losses of aromatic compounds are as follows in dependency on the contents of nonaromatic compounds in the starting materials:
______________________________________ Nonaromatic compounds Losses of in the starting aromatic Starting material material compounds ______________________________________ Coke oven benzene 4 weight percent 1.04 weight percent Pyrolysis gasoline 20 weight percent 6.25 weight percent Pyrolysis gasoline 30 weight percent 10.71 weight percent ______________________________________
The above figures indicate that the losses of aromatic compounds are more severe in the cases of higher content of non-aromatic compounds in the starting materials. It is desired to obtain low losses of aromatic compounds in the magnitude of from about one to three weight percent.
Therefore, employing extractive distillation for purifying starting materials containing large amounts of nonaromatic compounds is only of technical interest if it is possible to keep the content of aromatic compounds in the raffinate low and thereby to limit the losses in aromatic compounds. Calculations have shown however that variations in the amount of heat input into the extractive distillation column in the magnitude of 0.009 percent of the total heat input result in a variation of the content of aromatic compounds in the raffinate of about one weight percent. This illustration underlines clearly the necessity of providing an extremely accurate and well defined control of the heat input to the extractive distillation column.
It is usual in distillative separation processes to control the heat input to the reboilers at the bottom of the columns depending on the top and/or bottom temperatures. It is also conventional to maintain the thermal status of the column via changes in the amount of reflux and or reflux temperature. Furthermore the heat input to the reboilers of the distillation column has been controlled in the past depending on the amount of product at the top.
In particular in extractive distillation it has been proposed to control the amount of heat input to the extractive distillation column by changing the amount of extractant entering at the top into the extractive distillation column. This results in the disadvantage of continuously varying the ratio of extractant to starting material from the ratio providing an optimal separation, which should be employed at constant value for economic separation operations. Changing the amount of extractant entering at the top further results in lower yields and lower grades of purity of the product obtained. It has also been tried in practice to control the heat input to the reboilers at the bottom of the extraction column depending on the temperature sensed at one of the upper column plates, and the measured temperature value then serving for example for controlling the steam input of the reboiler.
As a matter of fact, the above indicated methods are insufficient for providing the accuracy of the heat input of the extractive distillation column which is required in practice for maximizing the yields.